1. Field of the Invention
The present invention relates to a bypass turbojet-engine cowling equipped with a thrust reverser comprising an electrically grounded displaceable component.
2. Description of the Related Art
As regards a bypass turbojet engine, it is known to fit the engine with a duct at the rear of the fan in order to channel the so-called cold, bypass flow. This duct comprises an inner wall enclosing the actual engine structure at the rear of the fan and an outer wall with an upstream portion continuous with the engine casing enclosing the fan. This outer wall is able to channel simultaneously the bypass flow and the primary flow at its downstream portion, namely to the rear of the exhaust of the primary, so-called hot flow in the case of a mixed-flow cowling for instance, or to channel only the bypass flow in the case of so-called separate-flow pods.
A wall furthermore may fair the engine outside, that is the outside of the casing enclosing the fan and the outside of the outer wall of the above discussed duct, in order to minimize powerplant drag. This applies especially to powerplants assembled to the aircraft's exterior, in particular when such powerplants are attached under the wings or affixed to the rear of the fuselage.
The thrust reverser relating to the present invention which provides flow deflection to generate thrust reversal is known per se and may be of the door/flap-type or of the cascade-type.
FIG. 1 of the attached drawings shows a known design of a so-called cascade-type thrust reverser used in a bypass turbojet engine wherein a rear portion of a cowling 7--which in the forward-thrust mode of the turbojet engine forms all or part of the downstream end of the outer wall of the annular duct of the bypass flow--is displaceable in slides in the downstream direction by a control system, illustratively a number of linear actuators affixed on an upstream portion of the thrust reverser. The rear cowling displacement causes a plurality of flaps 3 to pivot in order to seal the duct and to deflect the bypass flow into a reversed flow. The reversed flow is guided by a set of cascades 6 which are configured on an outer periphery of the duct and which, following the downstream displacement of the rear portion of the cowling 7, will be exposed.
Illustrative designs of these thrust reversers are found in European patent document A 0,109,219 and U.S. Pat. No. 3,500,645.
FIG. 2 of the attached drawings shows a known design of a door-type thrust reverser for a bypass turbojet engine wherein a plurality of doors 10, which form part of the outer wall, are driven to open a passage across the outer wall and to shut the annular duct rearwardly of the passage such that the bypass flow is guided outward towards the front of the cowling. Illustrative designs of such thrust reversers are found in French patent 1,482,538.
The displaceable parts, the door 10 or the rear portion of the cowling 7 in the above examples, are connected to a stationary portion of the cowling structure by intermediate components which may be swivels, when the displaceable part is driven into rotation, or may be a slide/slider assembly, in the case where the displaceable part is driven into translation.
In general, lubricants are prohibited anywhere on the cowling. In order to obtain a low coefficient of friction and to avoid eventual seizure of the intermediate components, a plastic cladding is placed at the contacting interfaces of the intermediate components.
This cladding electrically insulates the displaceable part from the remaining cowling structure.
Accordingly, in case of lightning, an arc striking the displaceable part cannot propagate unhampered toward the stationary structure which is connected to the grounding mass of the aircraft.
Several adverse effects which degrade structural reliability will thus result from a lightning strike. There being little assurance of conductance, or none, the current from the lightning strike will cause intense heating, in particular at the site of impact, thereby severely damaging the structure. The lightning current will flow and cause sparking at the interfaces of the intermediary components, damaging the plastic cladding and possibly wholly destroying it, thereby degrading the control of the displaceable parts.
Conventionally, electrical grounding is carried out to ensure electrical continuity. The displaceable and stationary components are connected by a braid which allows relative motion. This technique is well suited for limited lengths and specific orientations and preservation of a flexible connection; however, it introduces at least two operational constraints:
If the relative displacement is large, for instance in a cascade thrust reverser wherein the rear portion of the cowling is driven more than 500 mm, the flexible connection no longer fits; that is, the required displacement of this magnitude no longer allows proper housing of the braid; moreover the overall structural bulk is substantially degraded; PA1 In case of lightning, the braid is subjected to substantial electrodynamic stresses in the form of a whip action on the braid which is magnified by the substantial length of this flexible connection; this whip action may unacceptably damage the braid per se as well as the adjoining structure.